1. Field of the Invention
This invention relates generally to the field of pulsed power circuitry, and in particular to charging and regulating capacitor energy stores.
2. Description of the Prior Art
Pulsed power applications are based upon storing a quantity of energy in a capacitor bank by charging it from a prime power source. This energy is subsequently switched out of the capacitor bank in a shorter time than initially required to charge the capacitor bank, thereby achieving a higher peak power.
Circuits used to charge a capacitor store must have a current limiting feature to limit the surge current loading on the source. The nature of this current surge is twofold. One aspect of the surge is related to the capacitor, which, when totally discharged at the beginning of the charging process, is a short circuit. It will, therefore, draw a current from the source limited only by the impedance or other similar characteristics of the charging circuit. The other aspect of the surge is traceable to the starting phase relation of the power source voltage to the closure of the switch or contactor.
The first cycle saturation problem common to all ferromagnetic core transformers is a result of the fact that ferromagnetic core inductors including transformers will saturate if the flux density of the core exceeds the saturation level. For economic reasons ferromagnetic core devices are designed to operate slightly below saturation when the device is in steady state operation. However, when the device is first turned on, by having an AC voltage applied, the transient flux can exceed the steady state flux by a factor of approximately two. The occurrence of this high flux is statistical in relation to the precise time or phase during the AC voltage cycle at which the switch connecting the device to the AC source closes. If the device does not have a saturable core, or the flux does not exceed the saturation level, then the maximum surge is limited to about twice the steady state current. However, if the core is saturable, then typical current surges will be ten to twenty times the steady state value. These large surges can and do cause problems that have been dealt with in a number of ways. If the surge exceeds the circuit breaker trip level then it will open and will have to be reset before attempting a restart. Since the exact closing phase is random, the breaker will not open every time. Typically one might experience a random breaker opening about 5 percent of the time. Sometimes this one in twenty fault is simply tolerated and the procedure is to try again until the circuit holds.
Another method to eliminate the problem is to apply the voltage slowly, i.e., over a period of several cycles of the supply voltage. This provides for a slow build up of the flux in the core and prevents the saturation level from being exceeded. This method requires the use of additional and expensive components to accomplish the slow application of voltage.
A third method is to use two contactors (switches) to accomplish a "step start". The method employs an impedance that is in series with the power source to the device. One contactor is then used to connect the power source to the series impedance. The second contactor is placed across the impedance and is programmed to close a short time after the first contactor. When the first contactor closes the series impedance limits the surge current but permits the device to develop about half of the rated flux. When the second contactor closes, it increases the applied voltage to the device by a factor of about two to the rated input level. The step-start thus provided causes the flux to come up in two small steps with a settling period between rather than one large step with no settling time. The result is a much smaller surge which can be accommodated by the circuit breaker. The disadvantage of this the method is it requires twice as many contactors in addition to the surge limiting impedance. It is, however, less expensive than the equipment required for the previous method of a slow voltage run-up.
In addition to the current surge-limiting feature, the circuit must provide a feature that controls or regulates the maximum voltage to which the capacitor store is charged. This regulation feature must accommodate fluctuations in the prime power source and other parameters that may drift in time due to temperature variations or other factors.
The present state of the art approach for charging and regulating capacitive energy stores is the "switching regulator" type of power supply. The principle of operation consists of directly rectifying the AC main power source to obtain a DC source. The direct rectification avoids the use of a large expensive transformer, only relatively small rectifiers are required. The DC thus obtained is then switched at a high frequency, typically 20 kHz or more, into a small capacitor through an inductor, and then into the primary of a high voltage step-up transformer.
Solid state switching devices such as (Insulated Gate Bipolar Transistors) IGBT's or power (Monolithic Oxide Silicon Field Effect Transistors) MOSFET's are used. The high switching frequency permits the size of the transformer to be greatly reduced in relation to a transformer operating at the power line frequency, i.e. 20 kHz or higher compared to 60 Hz or 400 Hz. The secondary of the transformer connects through a high-voltage rectifier and then to the capacitive store. Each switching cycle of the circuit delivers a measured amount of energy to the capacitive store. The voltage on the capacitive store is sensed and compared to the desired full charge voltage. When that voltage is reached the switching is stopped, thus accomplishing the regulation function. In some variations of this type of switching regulator the frequency of the switching cycle is programmed in such a way as to maintain a more uniform power drain on the primary power line than would be obtained if the switching frequency were constant. Switching regulators work quite well but are complex and expensive. Since IGBTs and MOSFETs have limited power handling limitations, very large numbers of these components and associated circuits are required to handle high energy and/or high average power levels.